I have a 2008 JK 4 door X. It has the 3.8 (mini van) engine with an automatic transmission with 3.73 gears. I recently took off the stock 17" all terrain tires and put on a set of 17" stock Rubicon tires and rims I got from Ebay. The jeep runs a lot better with the overdrive button in the off position. What is the best rpm range for crusing on the interstate at 70 mph? My tach reads just under 2000 at 70mph. The engine seems to like it better at 2500-3000. Will this hurt the engine to run out of overdrive or does this little engine like the revs?
Personally, I would use the overdrive if you are just rolling down the interstate at 70mpg with no stops and starts like city driving.
I would think the overdrive would help mpg, and the engine and tranny are working less hard?
Just hit that OD button off if you need to pass someone or climb an incline?
stomping down on the pedal to pass someone with this engine is pathetic. It seems to shift down and then make alot of high revving noise and it really doesn't move you very well past that car.:thumbdown:
I think a big improvement on the JK would be if the OD button was on the column. That would make it a lot easier to switch the OD on or off. It is a real pain and safety hazard to keep reaching down for the button. I just got back from a trip to DC from Louisiana and I think the 3000 mile trip actually put 10,000 miles on the engine crossing the Smokies. What a bi-ch.
With my 5.13's and 35's I turn 2600 at 75 mph. I think that is about ideal for this engine. My understanding is that the best range for power and economy is 2400 to 2700 rpm. Before the 5.13's but with 35's it felt awful at 2050 rpm at 75. The stock Rubi (32/4.10) spins 2250 at 75 and that is, in my opinion for what little it is worth, too low. The Rubi should come stock with 4.56 gears.
I just ran the calcs on your setup and at 70 with OD off you should be at 2742. That won't hurt anything. This little engine likes revs.
Thanx guys, I think for most of my driving I will have the OD button off unless I am blasting down the interstate. Does anyone know if a gear change in the rear ends voids the warranty?
The OD on the auto is .69 while the 6-speed OD is .84. So it sounds like the 6-speeds do just fine and it seems the bulk of complaints are with the auto. So maybe the 6-speed Rubi shoud ship with 4.10's and the auto with 4.56's.
Its very hard to load an automatic jeep down below 2500rpm on a dyno, they will downshift. Plus that is wide open throttle and in open loop. your looking for light throttle closed loop highway speeds.
at 50 mph I get 17MPG @ 2200rpm
and get 12.5 MPG at 70 mph 2800 rpm
No matter what you do the faster you try to drive the harder it is to push the brick we drive thru the air. :thumbdown:
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